This invention relates to the manufacture of semiconductor devices and, more particularly, to etch masks and processing for forming grooves in semiconductor material.
Grooves are etched in semiconductor material for a variety of applications. In one case, V-grooves are etched in silicon substrates to form guides for aligning arrays of optical fibers. On the other hand, V-grooves are also etched into Group III-V compound semiconductors in the fabrication of various buried heterostructure (BH) lasers. For example, in one design of the channeled substrate buried heterostructure (CSBH) laser, as shown in FIG. 1, V-groove 24 bifurcates a blocking p-n junction in an InP substrate, and liquid phase epitaxy (LPE) is used to grow a double heterostructure (DH) of InP-InGaAsP-InP layers in the V-groove. The InGaAsP active layer 12 of the CSBH laser has the shape of a crescent, and the cross-sectional area of that layer must be accurately controlled in order to insure, inter alia, fundamental transverse mode, low threshold operation. These desiderata are, in turn, determined by the shape of the V-groove and the position of the active layer therein.
For example, fundamental transverse mode operation up to optical output powers of 10 mW/facet requires the width of the active layer to be .ltorsim.2.5 .mu.m. To achieve this dimension epitaxial growth in a sharp-bottomed groove is necessary. In addition, the grooves preferably are aligned parallel to the [011] direction on the (100) surface, in order that the side walls of the grooves are (111)B crystallographic planes. The (111)B planes prevent n-InP buffer layer growth on the groove wall, thus eliminating an undesirable leakage current path around the active layer.
The shape of the V-groove is a function of numerous parameters: (1) the particular semiconductor material, (2) the etchant and etching conditions, (3) the crystallographic orientation of the surface being etched, (4) the orientation of the etch mask, (5) the nature of the etch mask material, (6) the extent to which the etchant undercuts the mask, and so forth. Indeed, the groove oftentimes does not have the precise shape of a V at all, which for CSBH lasers renders it exceedingly difficult to control the dimensions of the critical active layer. FIGS. 2-4 illustrate the problem of etching a groove aligned parallel to the [011] direction on the (100) surface of InP. When essentially no undercutting of the etch mask occurs, as shown in FIG. 2, the bottom of the groove has the desired V-shape with oblique (111)B walls, but the top of the groove disadvantageously has vertical (011) walls. In contrast, for the same size mask opening w and etching depth D, when excessive undercutting occurs as shown in FIG. 3, the groove has the desired (111)B sidewalls, but the bottom of the groove is flat [a (100) plane] rather than pointed and the groove is much wider. On the other hand, if undercutting is carefully controlled, as shown in FIG. 4, the groove has the proper V-shape with a pointed bottom and only (111)B side walls.